1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to a system for imaging of recording media such as lithographic printing members.
2. Description of the Related Art
Imaging devices that utilize laser power sources frequently deliver the output of the laser to its destination using an optical fiber arrangement. This frees the designer from the need to physically locate the lasers directly adjacent the recording medium. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 (the entire disclosures of which are hereby incorporated by reference) disclose the use of lasers to impress images onto lithographic printing-plate constructions. As described in these patents, laser output can be generated remotely and brought to the recording blank by means of optical fibers and focusing lens assemblies.
It is important, when focusing radiation onto many types of recording medium, to maintain satisfactory depth-of-focus--that is, a tolerable deviation from perfect focus on the recording surface. Adequate depth-of-focus is important to construction and use of the imaging apparatus; the smaller the working depth-of-focus, the greater will be the need for fine mechanical adjustments and vulnerability to performance degradation due to the alignment shifts that can accompany normal use. Depth-of-focus is maximized by keeping output beam divergence to a minimum.
Optical efforts to reduce beam divergence also diminish power density, since a lens cannot alter the brightness of the radiation it corrects; a lens can only change the optical path. Thus, optical correction presents an inherent tradeoff between depth-of-focus and power loss. U.S. Pat. No. 5,822,345 discloses an approach that utilizes the divergent output of a semiconductor or diode laser to optically pump a laser crystal, which itself emits laser radiation with substantially less beam divergence but comparable power density; the laser crystal converts divergent incoming radiation into a single-mode output with higher brightness. The output of the laser crystal is focused onto the surface of a recording medium to perform the imaging function.
The arrangements described in the '345 patent employ a separate crystal for each diode pumping source. This is ordinarily necessary due to the nature of laser crystals and their operation. In the absence of optical excitation, resonant cavities formed from these optical-gain crystals are flat-flat monoliths; when optical power is delivered to an end face of such a crystal, however, this and the opposed face bow--an effect called bulk thermal lensing. To obtain a single transverse mode of operation (preferably the lowest-order, fundamental TEM.sub.00 mode), with the output divergence as close as possible to that of a diffraction-limited source, the crystal must be implemented in a design that accounts for bulk thermal lensing.
This phenomenon makes it even more difficult to obtain multiple, independent outputs from a single laser crystal. Even if the energy of each pumping source is confined to a discrete region on one of the crystal faces, the thermal lensing action required for lasing in one region will ordinarily affect the other regions, resulting in mutual interference. This condition is known as "thermal crosstalk." Accordingly, the current state of the art prescribes the use of a separate crystal for each laser channel, resulting not only in added cost for the crystals and their mounts, but also for separate focusing assemblies.
In addition, the configurations described in the '617 and '092 patents (and, somewhat more pertinently, in U.S. Pat. No. 5,764,274) contemplate permanent affixation of the diode laser packages to the optical fiber. This is due to the need for efficient coupling of the laser energy into the end face of the fiber. Components are therefore permanently joined so that the alignment therebetween remains undisturbed during operation. Should a diode fail, not only the diode but the entire optical-fiber assembly must be replaced. Such a requirement is of little consequence in the arrangements described in the '274 patent, since the the fiber is coupled to a focusing assembly using an SMA connector or the like, which is conveniently removed and replaced. In arrangements having fiber outputs that are less accessible or which require more involved mounting operations, however, permanent diode affixation at the input side of the optical fiber can prove decidedly disadvantageous.